We propose to evaluate several hypotheses examining the role of prolactin in postmenopausal breast cancer etiology, which build upon our previous findings that prolactin levels are positively associated with risk of invasive postmenopausal breast cancer, with a particularly increased risk for estrogen receptor positive tumors. Substantial in vitro and animal data support the role of prolactin and the prolactin receptor in breast cancer development, specifically by promoting cell proliferation, increasing cell survival, altering cellular motility, and supporting tumor vascularization. Analyses will be conducted using data from the Nurses' Health Study, which began in 1976, using women who provided a blood sample in 1989-1990 (~33,000) and also focusing on a subset of breast cancer cases (~500) for whom we have both a blood sample and tumor tissue blocks. Specifically we will extend our findings by examining the role of genetic variability with respect to both prolactin concentrations in the blood and breast cancer risk, using germline DNA obtained from white blood cells. We will also provide the first analysis examining whether prolactin receptor status in breast tumors modifies the association between prolactin concentrations and breast cancer risk, using our unique population-based tissue bank. In addition, we propose to expand information concerning the relationship between prolactin levels and various lifestyle and other breast cancer risk factors, particularly focusing on reproductive factors, measures of energy balance and diet, early life exposures, and family history of breast cancer. These analyses will help us to better understand the regulation of prolactin levels in plasma as well as provide potential mechanistic information on how these factors may modify risk of breast cancer. Finally we propose to conduct important and necessary pilot testing for two assays that will provide preliminary data needed to pursue these as biomarkers in a future grant. Major strengths of this application are the prospective collection of breast cancer cases and the availability of questionnaire data, archived plasma and DNA samples, and tumor tissue samples from our breast cancer cases. The proposed analyses will improve our understanding of the role of prolactin and the prolactin receptor in postmenopausal breast cancer etiology. This potentially may affect public health by increasing our understanding of how breast cancer develops, which in turn can lead to a better understanding of how to prevent this disease. [unreadable] [unreadable]